1. Field of the Invention
This invention relates to heat curable organosiloxane compositions. More particularly, this invention relates to organosiloxane compositions exhibiting a useful pot life at ambient temperature and the ability to form cured sponge rubber when heated.
2. Background Information
Organosiloxane compositions useful for forming silicone sponge rubber can be classified into two categories based on the mechanism used to generate the gas which forms the sponge. One category of compositions lacks a mechanism to generate a gas as part of the curing reaction, and therefore contains a heat activated foaming agent. Compositions of this type are disclosed in Japanese Pat. No. 69-461 and Japanese Unexamined Applications Nos. 75-50465, 76-46352, 77-81378 and 83-79030. In the second category the gas is generated as a by-product of the curing reaction. Compositions of this type are disclosed in Japanese Pat. Nos. 58-9297 and 70-9497, Japanese Unexamined Application Nos. 77-16564, 78-36556 and 83-40348 and U.S. Pat. No. 3,428,580, which issued to Nietzsche et al. on Feb. 18, 1969.
Japanese Unexamined Application No. 77-16564, published on Feb. 7, 1977, discloses foamable compositions prepared using a vinyl substituted dimethylsiloxane polymer, an organohydrogensiloxane containing at least two silicon-bonded hydrogen atoms per molecule, wet processed silica and a platinum catalyst.
Compositions corresponding to the aforementioned second category typically exhibit a short time interval between combining of all ingredients and foaming and curing of the resultant reaction mixture at ambient temperature. This interval is typically referred to as "pot life." In accordance with the teaching of U.S. Pat. No. 3,923,705 that issued to Smith on Dec. 2, 1985, a small quantity of platinum catalyst inhibitor such as a cyclic methylvinylsiloxane or an acetylenic alcohol can be added to extend pot life. When these inhibitors are used to extend the pot life of a foamable composition beyond ten minutes, the resultant sponge rubber exhibits the disadvantages of large cell size and reduced uniformity.
U.S. Pat. No. 4,492,775 that issued in the name of Koshii et al. on Jan. 8, 1985 teaches foamable compositions containing a hydroxyl-substituted polyorganosiloxane, an organohydrogensiloxane, a platinum catalyst, a platinum catalyst inhibitor and a finely divided filler in combination with (1) an organosilicon compound containing at least 0.5 weight percent of carboxyl groups or silicon-bonded hydroxyalkyl groups, or (2) a partially esterified polyfunctional alcohol. In addition to being nonflowable, the pot life of the compositions is extended without adversely affecting the structure of the cured foam.
An objective of this invention is to provide a method for extending the pot life of foamable compositions containing an alkenyl-substituted polyorganosiloxane, an organohydrogensiloxane and a platinum catalyst in the absence of any hydroxyl containing compounds other than atmospheric moisture. A second objective of this invention is to provide polyorganosiloxane compositions exhibiting commercially useful pot lives. When heated the compositions yield cured sponge rubber exhibiting a uniform distribution of small cells.